Framebuffers previously assumed a readPixels buffer alignment
of 4 bytes, which causes issues when attempting to read with
a format larger than 4 bytes per pixel. Change the default
alignment to match the size of the largest know Vulkan format

Bug: angleproject:3731
Change-Id: I6cd26a99e6f3b2f30084d403630c17f51a0ecedf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1727731
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>